Spark ignition of an air/fuel mixture within a combustion chamber of an internal combustion engine typically involves igniting the air/fuel mixture with an electric spark jumped between an electrode and a ground electrode of a spark plug. An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. No. 3,921,605 to Wyczalek and U.S. Pat. No. 4,924,829 to Cheng et al., offers several advantages over spark ignition approaches. As the name suggests, torch jet-assisted spark ignition utilizes a jet of burning gases which is propelled into the combustion chamber in order to enhance the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio.
In a torch jet-assisted spark ignition system, the jet typically emanates from a combustion prechamber, and passes through an orifice into the main combustion chamber. Though an air/fuel mixture can be introduced directly into the prechamber through a separate intake valve or fuel injector, it is generally preferable that the air/fuel mixture originate from the main chamber in order to simplify the construction of the engine and its ignition system. Furthermore, combustion of the air/fuel mixture within the prechamber can be initiated from within by a separate igniter, or can be initiated by the flame front within the main chamber. With either approach, combustion typically proceeds relatively simultaneously in both the prechamber and the main chamber. However, because of the small relative volume of the prechamber, a high pressure is developed in the prechamber while the pressure is still relatively low in the main chamber. As a result, a jet of burning gases shoots from the prechamber far into the main chamber, and thereby significantly increases the combustion rate in the main chamber.
Engine testing of torch jet spark plugs in accordance with U.S. Pat. No. 4,924,929 to Cheng et al. has verified that torch jet-assisted ignition results in faster burn rates than conventional spark ignition techniques. Yet, further enhancements in performance characteristics and capabilities, as well as improvements in manufacturability, would be highly desirable. Such improvements include a greater resistance to pre-ignition of the air/fuel mixture due to high operating temperatures. In particular, internal electrodes used to ignite the air/fuel mixture within the prechamber of a torch jet spark plug may become sufficiently hot to prematurely ignite the air/fuel mixture, creating the potential for both damage to the spark plug as well as to the engine. Another desirable improvement would be a construction which would eliminate the possibility of internal short circuits to ground forming within the spark plug as a result of deposits building up on the mating surfaces of the inner electrodes.
Thus, what is needed is a spark plug for a torch jet-assisted spark ignition system, wherein the spark plug is capable of generating a jet which increases the burning rate within a combustion chamber of an internal combustion engine, while also being resistant to pre-ignition and short circuiting. Yet, in order to assure that the spark plug is practical for mass production, the spark plug must also be relatively uncomplicated in its structure and construction so as to be relatively easy to manufacture.